1. Field of the Invention
The present invention relates to a programmable sequence controller using a technique called microprogramming.
2. Description of the Prior Art
In a programmable sequence controller having an operation processing section of the random logic type, the circuits have become complicated.
To minimize this drawback without slowing the execution speed of a sequence program, it has been considered to use a technique called microprogramming in a programmable sequence controller. In such a sequence controller, the operation processing section thereof is generally provided with a memory for storing a microprogram including a plurality of microinstructions, a memory address control unit for designating a memory address of the memory so as to read out a microinstruction therefrom, and a central processing unit (CPU) for performing data processing operations in accordance with the read-out microinstruction. However, since the central processing unit (CPU) which is commercially available is designed for use in a general-purpose computer, it has too many functions to be used in a programmable sequence controller and is relatively expensive. This results in a high cost programmable sequence controller.
Further, it has been suggested to use a commercially available microprogram control unit (MCU) instead of the above-mentioned central processing unit (CPU). In such a system, a sequence control operation is executed by utilizing test flags provided in the microprogram control unit (MCD). More specifically, the test flags are caused to be set or reset in accordance with the data included in the microinstruction and the ON/OFF state of an I/O element, so that the memory address of the memory is designated thereby to read out a microinstruction which is to be executed next. Such a system is disclosed in U.S. Pat. No. 4,249,248. However, the microprogram control unit (MCU) also has excess capabilities, such as various jump functions to alter a designated memory address to a remotely-located memory address, if used in a programmable sequence controller in which the designation of a memory address is carried out rather simply. The microprogram control unit (MCD) is also relatively expensive and, as a result, the programmable sequence controller cannot be built inexpensively.
Furthermore, to overcome the above disadvantages, it is conceivable to use a counter to designate memory addresses, as known in the field of general purpose digital computers. However, sequence control operations cannot be executed if the memory addresses are orderly designated by the counter, because it is necessary to read out the microinstructions from various memory address in accordance with the ON/OFF states of the input and output elements.